On December 26, 2004, at 7:58 a.m., the third largest earthquake ever recorded by seismographs occurred 160km off the northeast coast of the island of Sumatra. The earthquake was rated between magnitude 9.1 and 9.3 and broke the record for duration: 8 to 10 minutes. Seismologists determined that the epicenter lay at a depth of 30km below the surface of the ocean.
This violent earthquake triggered a tsunami, a high-speed water wave of terrifying power that spread in all directions from the epicenter. The eastern side of the wave roared across the short distance to the coast of Sumatra as a wall of water 10m high. The western side ran out into the Indian Ocean at a speed of about 720km/h. It reached Sri Lanka and eastern India a couple of hours later, then swirled around to hit Phuket in Thailand. Finally, 7 hours later it swept across the Horn of Africa.
The coastal wave swept inland in Sumatra, destroying everything in its path. The four main towns in the province of Aceh, each home to as many as 12,000 people, disappeared within minutes. Of 28 villages along the coast, only 4 survived. At the north-facing city of Banda Aceh, the oncoming wall of water was 11m high. At the west-facing villages of Meulaboh and Tapaktuan, the wall towered an astounding 30m (98ft) high. In fact, the wall measured between 15 and 30m high for 100km along the coast, from Kreung Sabe to the northwest tip of Sumatra. The water poured inland for 10km and more, bulldozing houses, cars, and trees, tearing up roads and bridges, pulling down power lines.
A TV reporter broke down in tears as she saw the devastation at Banda Aceh. “The city’s like a huge dumpster,” she said. “Dead bodies still scattered everywhere, debris blocking the streets with possibly victims of the tsunami in those piles. Help hasn’t arrived yet even there in the capital of Aceh. People who are hungry tried to break in one grocery store only to find food covered in mud and water and bodies of people who couldn’t escape at the time the tsunami hit” (BBC News, Jan. 6, 2005).
In Phuket, Thailand, 2,300km from the epicenter, a 14-year-old girl gave the following eyewitness report, printed in the Guardian (Dec. 22, 2006):
Dad is looking out to sea in a strange way. Mum and I look up and see the water disappear, leaving all the fish on the sand. We see children running out to help the fish back into the water, so that they do not die. Dad wants me to fetch the camera from the hotel so that we can film the water disappearing. I am too lazy. Dad gets up to fetch it himself, but first he and Mum have a little argument. Dad thinks that the water is drawing out. Mum and I shriek, “It’s coming in.”
“Calm down—of course it isn’t coming in,” says Dad, on his way to the hotel. I have not seen him since.
Mum and I see the wave. We take our stuff and run. Mum runs away ahead of me. I hear her voice: “For goodness sake run, Charlotte! Whatever happens I will always love you.” I have not seen her since.
She disappears without bothering to check whether I am behind her. I run in panic, upwards, as far as I can. Get to a flight of steps where there is chaos. A small child is standing by the steps crying. The mother has left the child alone.
I am holding tightly on to the stair rail when the wave roars in over the whole of Khao Lak. I feel the wave rolling over me and pulling away the rail. I go with the wave out to sea and in again, several times. Under the surface, I swallow gulps of salty water when I try to get air. I will not survive if I do not come up to the surface. In the end I can take deep breaths. With my eyes closed. I am hanging in something, a tree? The roof of a house? The thing I am hanging on snaps and I am pulled out to sea again, out and in. After perhaps seven minutes I open my eyes. I have landed up by the hotel and see masses of people lying there, blood everywhere.
The girl’s brother was watching a film in the hotel when the wave hit. Here is his report:
I lay down on the bed to watch “The Day After Tomorrow,” a film about an enormous wave. Quite weird it was that film. Mum, Dad and Lotti went down to the beach.
The electricity is cut off. There is a sudden sound like thunder and heavy rain. I look out of the window—and see a wave that must be 15m high. I see bungalows being swept away, and cars, and people lying bleeding, and I understand nothing. I grab the room key and want to run down to the beach, but all the paths and steps have gone. Get very worried for Mum, Dad and Lotti. See people floating in the water and start crying.
The children never saw their parents alive again.
In Sri Lanka approximately 35,000 people died; in the eastern and southern coasts of India, the toll was 9,000; in the Andaman and Nicobar Islands, 5,000 died. In Thailand, a three-story wave hit hotels and villages, killing 5,000. In Somalia, 5,570km from the epicenter, entire villages were swamped and 300 died. Many aftershocks were recorded at the epicenter for several days. One of them had a magnitude of 8.7 and could be considered a “triggered earthquake.” None of these aftershocks produced a severe tsunami, however.
When the dead and missing were counted, weeks after the disaster, the toll had reached a staggering 230,000, spread over 14 countries. The great majority (137,000 dead and 37,000 missing) were in Indonesia, where the wave hit hardest and without warning. In addition, over half a million people were displaced, and millions were left homeless. A massive international rescue effort was mobilized to help the survivors, but eight years after the event, the people of Aceh province are still trying to resume their former lives.
Fortunately, tsunamis as powerful as this 2004 event are rare. They are generated primarily by submarine earthquakes but can also be caused by undersea landslides and volcanoes. Submarine landslides can also generate huge tsunamis. While tsunamis associated with earthquakes are limited in height by the magnitude of the quake, a landslide tsunami is limited only by the vertical distance of the slide. An underwater slide can begin on the continental slope at a depth of a hundred meters and end at a depth of several thousand meters. The gravitational energy released in such a slide can create a monster tsunami.
The most common cause of a tsunami produced by an erupting volcano is a pyroclastic flow of superheated gases and rocks that has erupted from a volcanic vent and either flows into the sea at very high speeds or bursts out from a submerged vent. In either case, the pyroclastic material displaces huge volumes of water, which bulge up into a gigantic wave.
Tsunamis caused by landslides and volcanoes can often be far larger than earthquake-caused tsunamis. Tsunamis can also be triggered by calving glaciers and meteorite impacts, although these are generally smaller than landslide-caused tsunamis.
In the Pacific Ocean the sources of the earthquakes (and of many volcanic eruptions) lie in a narrow band around the perimeter, the so-called Ring of Fire. It touches the whole western coast of North and South America, the Aleutian Islands, the east coast of Asia, Indonesia, and the chain of islands that terminates at New Zealand. This band marks the boundaries between colliding tectonic plates.
As you may recall, the crust of the earth is divided into approximately 18 tectonic plates of varying sizes, each about 100km thick. The plates move horizontally in different directions, a few centimeters per year. They are driven by slowly turning convection cells in the underlying molten mantle of the earth.
Where two plates meet, they may collide head-on and raise a ridge of rock that forms a mountain chain. (The Himalayas, for example, are formed by the collision of the Indian and Eurasian plates.) Two plates may diverge so that hot lava seeps up through the crack between them, quickly cooling to form new plate material. (The mid-ocean ridges that wind around the globe are formed this way.) Alternatively, plates may slide past each other, as happens on the San Andreas fault, along the west coast of North America. Finally, one plate may submerge beneath the other, a process called subduction. Subduction is the main type of plate interaction for the Ring of Fire.
In subduction, the plate motions need not be smooth. The plates may jam so tightly that an enormous stress builds up in the rock at the junction. At some critical point, the logjam breaks, and one plate shifts violently and thrusts suddenly beneath the other.
That is what happened in the Indonesian earthquake. The Indian tectonic plate (part of the Indo-Australian plate) thrust under the Burma tectonic plate (part of the Eurasian plate) at the Sunda trench, which stretches along the western coast of Sumatra. A section of this Sunda trench, assumed by many geologists to be dormant, ruptured violently along a length of 1,600km. It slipped sideways about 15m at a speed of 2km/s, like a crack opening on a frozen lake. The seafloor on the overriding Burma plate uplifted seaward (toward the trench) and downward toward the coast, by about 4m in just seconds.
This vertical movement caused the tsunami. The whole water column, from the bottom to the surface, and over an area of 100,000km2, was lifted. The water rose into a great dome within a few seconds. Geologists estimate that the dome contained 30km3 of water. When the dome collapsed under its own gravity, it pressed against the water to the sides and launched a water wave that carried away the potential energy of the dome. This was the tsunami.
As a result of the 4-m vertical motion, an N-shaped water wave was created. The wave immediately split into two N-shaped waves, traveling in opposite directions. One wave headed west to the Indian Ocean with a leading crest. The other wave headed east toward the coast of Sumatra with a leading trough. Each of these two tsunamis propagated as a shallow-water wave, whose speed is determined by the local depth of the bottom.
The trough running toward shore encountered shallower water, and therefore slowed down. Its wavelength decreased as well. To conserve its energy, the wave’s amplitude increased, and a towering crest was created. In contrast, the seagoing wave was crossing the 4-km-deep Indian Ocean at a height of a few meters at most, and with a wavelength of tens of kilometers. It would have been hardly noticeable to a ship at sea. It wasn’t until it reached Sri Lanka and the Indian coast that the crest also rose to terrifying heights, causing tremendous loss of life a couple of hours after the earthquake. Some of the tsunami’s energy reached South Africa and even spread into the Atlantic and Pacific Oceans. Tiny tsunamis were registered there, possibly funneled by the mid-oceanic ridges, to reach South America and even Vancouver in Canada.
When the crest of the tsunami arrives at a shore, it builds to a great height. Most tsunamis run up a shore like a rapidly rising tide (which is why they are sometimes called tidal waves), but a few are affected enough by the undersea reefs and the slope of the beach to become breaking waves. The Indonesian Ocean tsunami was one of these few; it arrived as a breaking wave, similar to the surf that one commonly sees at a beach. Much of the damage the tsunami inflicted was caused by the swirling currents, loaded with the debris that it generated.
Most tsunamis (58%) are produced by earthquakes where tectonic plates collide. But not all submarine earthquakes produce tsunamis. Russian geologist Victor Gusiakov has looked into the matter. The main requirement is that the earthquake vertically displace the overlying water; slip-strike earthquakes (those sliding sideways) do not create tsunamis. In addition, as you might expect, the greater the magnitude of a quake, the higher the chance that it will produce a tsunami. Gusiakov showed that a quake as large as magnitude 6.7–6.9 has only a 12% chance of launching a tsunami, while only half of the quakes with magnitude 7.0–7.4 produce a tsunami. Earthquakes above magnitude 8 all produce tsunamis, generating 40% of the large transoceanic tsunamis, although not all have such devastating impacts as the 2004 Indian Ocean tsunami.
Whether an earthquake will generate a tsunami and how destructive that tsunami might be are difficult to predict. For one thing, it is not immediately clear how much vertical displacement of the water might have occurred. Secondly, the magnitude of a quake has very little relation to the energy of the tsunami it may produce because the total earthquake energy is related to the total ruptured area, not just to the maximum intensity at the epicenter. For example, the tsunami that a magnitude 7 quake produces can vary in energy by a factor of a million or more, depending upon the type and depth of the earthquake and the area of the rupture.
As a case in point, the earthquake of May 22, 1960, at Valdivia, Chile, is still the largest earthquake ever recorded, with a magnitude of 9.5. It generated a tsunami that battered the coast of Chile with waves as high as 25m. Houses were pushed 3km inland by the water. The tsunami crossed the Pacific and killed 61 people in Hilo, Hawaii; 138 in Japan; and 32 in the Philippines. It completely destroyed the port of Hilo and caused extensive damage on the west coast of the United States. Waves as high as 10m struck Honshu, Japan, 22 hours after the quake. But despite the 9.5 magnitude of this earthquake, the resulting tsunami had significantly less energy and caused only a fraction of the total destruction and death resulting from the (slightly) smaller Indian Ocean earthquake.
The degree of destruction and death caused by a tsunami also depends on the nature of the coastline, the steepness of the shore, the nature of the seabed, the proximity to the epicenter—and the awareness of the population to the potential danger from tsunamis. For instance, after the 2004 Indian Ocean earthquake, the aboriginal population of the Andaman Islands recognized the signs of the approaching tsunami and fled to safety, while their more “civilized” neighbors perished in the waves. Even 10-year-old Tilly Smith, playing on the beach at Phuket, Thailand, recognized the receding water as the onset of the tsunami and was able to get her family and others on the beach to safety.
Sometimes the aftereffects of an earthquake and tsunami are just as devastating as the original event. This was the case with the magnitude 9.0 quake that struck the east coast of Japan at 2:46 p.m. local time on Saturday, March 11, 2011. It was the largest quake ever recorded in the long history of Japanese earthquakes.
The epicenter lay 100km off the coast, at a depth of 6km. There, the Pacific tectonic plate had been driving down under the Eurasian plate for centuries. Stress in the rocks kept building and building. On March 11, the fault snapped. The seabed jolted upward 5–8m along the 480-km fault line, raising a gigantic volume of water in a dome a few meters high. Moments later, as the dome collapsed, it generated an immense tsunami that contained nearly double the energy of the Indian Ocean tsunami—enough energy to power the city of Los Angeles for a year.
Because of its huge wavelength, the tsunami propagated as a shallow water wave, whose speed was determined by the depth of the seabed. Part of the wave moved inexorably toward the shore, averaging over 100km/h, and part of the wave raced out across the Pacific toward Hawaii and the west coast of North America, at closer to 800km/h.
As the tsunami wave got closer to the Japanese coast, the front of the wave slowed even more because of the increasingly shallow shore, and the water at the rear piled up into a towering wave. About 30 minutes after the earthquake, the first wave hit the city of Ofunato as a wall of water probably 10–20m (34–68ft) high (analyses of these heights vary tremendously), sweeping everything before it. Down the coast, the city of Sendai was inundated next. The land around the city was dead flat, so that nothing stood in the way of the raging wave (fig. 9.1). A roiling mass of water and debris flooded inland as much as 7km. The surge climbed hills higher than 40m.
Somewhat farther away from the earthquake’s epicenter, the citizens of Miyako had a 60-minute warning to seek high ground. Years before, they had built a 10-m-high seawall to protect their city from just such calamities. Therefore, assuming they were well protected, about 40% of the people ignored the warning. But to their shock and horror, a 40-m tsunami roared over the wall easily, surged across the coastal road, and drowned the city (fig. 9.2). Boats, cars, trees, and small buildings were washed away like toys. Half of those who disregarded the warning perished in the powerful churning waters. Later, after the danger had passed, geologists learned that the earthquake had dropped the shoreline by half a meter, allowing the tsunami even easier access to the city.
Fig. 9.1 An aerial view of the Sendai airport on the coast of Japan on March 11, 2011. (Wikipedia.)
More than 19,000 Japanese were counted as missing or confirmed dead in the catastrophe. They were either swept out to sea or drowned. Farther to the east, the tsunami also caused serious damage in Hawaii and in some places along the western coast of the United States. Thousands of birds nesting on Midway’s low-lying islands were swept to their deaths. But except for one unlucky California man (attempting to photograph the tsunami), no humans outside of Japan lost their lives.
However, the monster tsunami created another serious threat to the population: the inundation of the Fukushima Daiichi nuclear power station. The plant had six reactor units, two of which were in cold shutdown for routine maintenance, while a third one had had its fuel rods removed in preparation for refueling. When the earthquake struck, the remaining three reactors automatically scrammed (shut down) exactly as they were supposed to. Power to the Fukushima plant was also severed as a result of the earthquake. Emergency generators clicked on immediately to maintain the flow of cooling water through the hot reactor cores.
Fig. 9.2 A view of Miyako on March 11, 2011. (Wikipedia.)
But then 50 minutes later, the 15-m tsunami arrived, easily topping the seawall and completely flooding the underground chamber where the generators were located. Unfortunately, this chamber had not been sealed, nor had the generators been moved to higher locations, as recommended by Japanese nuclear safety regulations. The generators drowned, the pumps that cooled the reactors shut down, and as the cooling water started to boil away and expose the nuclear core, the reactors heated up rapidly. Nothing was happening as it was supposed to anymore—the plant engineers were operating in unknown territory.
Officials realized that without cooling water, the reactor cores could melt down and cause a nuclear disaster comparable to Chernobyl. As a last resort, knowing it would destroy the reactors, they ordered seawater to be pumped onto the reactors in a desperate attempt to provide some cooling. First, they used fire truck pumps, but they eventually got three seawater pumps repaired from the tsunami damage, while crucial electrical equipment to run them was found in good shape on higher ground, as the regulations required. Nonetheless, the pressures within the reactors continued to build, and the hydrogen gases created by chemical reactions with the reactor material eventually caused explosions at all three reactors over the next couple of days.
Additional radioactive gases were released to reduce the pressure. But it finally became clear that all three reactors had experienced partial meltdowns. The fuel rods had become exposed to air and had heated up so much that they melted the bottom of the reactors and fell to the concrete floor below. The high levels of radioactivity measured at the plant hampered actions by the plant operators. More than 200,000 residents who were within 20km of the plant were hurriedly evacuated from the vicinity.
Not even reactor 4 escaped the disaster, although it was shut down. On Tuesday, three days after the tsunami, a fire broke out on the roof of this reactor, where its spent fuel rods were cooling, melting some of the rods and releasing even more radioactivity into the atmosphere.
The key to the emergency was the lack of power to cool the reactors. In the following weeks, workers risked their lives to build a power line into the plant. Eventually the power line was completed, power and cooling was restored, and the immediate threat of total nuclear meltdown was removed. But the radiation level was a thousand times normal within the plant and eight times normal in the surrounding countryside. It had poisoned hundreds of square kilometers of land, crops, and buildings, while several hundred thousand residents of the area lost their homes with no hope of ever returning. Many had lost everything they owned—their homes, their clothing, their pets. How does one cope in such a situation? How does the government grapple with the most immediate problems?
The government mobilized 100,000 troops to help in the relief effort. Nations around the world contributed to the relief of the stricken Japanese. Following the disaster, the Japanese government announced its decision to close down its nuclear power plants, in recognition of the public’s revulsion and fear. Recently, some plants are being restarted because Japan lacks viable alternative energy sources.
As with so many accidents, it was the unforeseen sequence of events that caused the worst of the devastation. Although the catastrophe was triggered by a natural disaster, one failure after another created havoc. Electricity failed, cell phones failed, trains stopped running, roads were impassible, and the radiation limited access to critical areas. However, human errors and misjudgments exacerbated the situation. One of the key misjudgments was the complacency of so many people that anti-tsunami seawalls could eliminate the risk from these hellish waves. The enormously energetic surge just washed right over them, knocking them down like children’s blocks. Those living near the ocean need to learn from their ancestors, who planted stone markers on hills showing where tsunamis had reached in their day—some still many meters above the reach of the March 2011 tsunami.
Japan has probably suffered more tsunamis than any other nation, averaging one about every 6 years. The first recorded tsunami occurred at Hakuho on November 24, 684 CE. The preceding earthquake has been estimated at a magnitude of 8.4. There followed a sequence of notable earthquakes and tsunamis, in 869 and 887. In 1293 a 7.1 magnitude earthquake and a tsunami struck the city of Kamakura, killing 23,000. Disaster struck again in 1361 with an 8.4 magnitude quake and again in 1498 with a 7.5 magnitude quake that left 30,000 dead. On February 3, 1605, an 8.1 magnitude quake triggered tsunamis with heights of 6–8m and caused more than 5,000 deaths by drowning. On October 28, 1707, an 8.4 quake triggered a tsunami with a reported height of 10m. Nearly 50,000 were killed. But none of these could compare with the Indonesian event of 2004, with a death toll of 230,000.
Deadly tsunamis are not limited to the Pacific Ocean. The Atlantic Ocean and Mediterranean Sea are also famous for such disasters. The earliest geological record is of a landslide-caused tsunami in the Norwegian Sea about 6100 BCE; scientists estimate that it deposited material from the sea at least 80km inland.
Around 1600 BCE a great volcano erupted on the island of Santorini in the Aegean Sea. The whole top of the island disappeared, leaving a semicircular caldera that now forms a tranquil bay. The eruption occurred at the height of the Minoan civilization, based on the island of Crete, which lies some 100km from Santorini. Recent research suggests that a powerful tsunami inundated the coast of Crete, killing thousands and leading to the decline of the Minoans. In 373 BCE, the Greek city of Helike, 2km from the sea, was struck by a tsunami and remained permanently submerged. It may have been the source of the legends of Atlantis.
On July 21, 365, a powerful earthquake triggered a monster tsunami, reportedly 30m high, that devastated the ancient city of Alexandria. Thousands were killed when, after the water suddenly retreated from the shore, they raced to capture the flapping fish that had been stranded in the slime. Just as suddenly, they were overtaken by the tsunami as the surge swept back in, scattering ships and bodies far inland. The disaster was described in vivid detail by a Roman historian, Ammianus Marcellinus.
One of the most famous earthquakes and tsunamis occurred on November 1, 1755, off the coast of Portugal. The capital city of Lisbon first was devastated by the earthquake and by the fires that broke out afterward. Tens of thousands fled to the waterfront to escape the fires. Forty minutes later, they were awestruck to see the sea recede, stranding ships in the harbor. Then a great wave, estimated at 15m in height, broke over the people, and flooded the shore for kilometers inland. The total loss of life from all causes could have been as high as 100,000, making this one of the deadliest earthquakes in history and certainly the greatest catastrophe of the century in Europe.
In August 1883 a series of spectacular volcanic eruptions occurred on the island of Krakatoa (Krakatau), which lies between Sumatra and Java in the Sunda Strait. Four explosions ejected several cubic kilometers of ash into the atmosphere and were heard 3,500km away in Perth, Australia. Tsunami waves as high as 30m were generated by the pyroclastic flows of superheated gas and rocks that fell into the sea. The town of Merek, on the northwest tip of Java, was obliterated by a tsunami reportedly 46m high. The waves swamped large areas of the coast of Sumatra and traveled as far as South Africa.
The eruption of Krakatoa also had other major impacts on the earth’s climate. The ejection of massive amounts of sulfur dioxide into the atmosphere led to the formation of sulfuric acid, which then promoted the formation of dense cirrus clouds. High-level winds carried the cirrus around the globe. These white clouds reflected sunlight more efficiently and caused a global drop in temperature of about 1.2°C. The cooling effect lasted for several years.
One of the largest recorded tsunamis was caused by an underwater landslide at Unimak Island, the easternmost island in the Aleutian chain, on April 1, 1946. The slide, triggered by a magnitude 8.6 earthquake, contained about 200km3 of sediments. It started at a depth of 150m and ended on the Aleutian Terrace at a depth of 6,000m. Several tsunamis were generated, ranging in height from 15m to an astounding 43m (141ft). A 35-m wave destroyed the lighthouse at Scotch Cap on Unimak and killed all five lighthouse keepers. Another wave crossed the Pacific, reached Hilo, Hawaii, after five hours and caused 159 casualties, as well as tens of millions of dollars of property damage. This tsunami is sometimes referred to as the April Fools Day tsunami because many people in Hawaii thought the tsunami warning was just a prank. It did result in the establishment of the Pacific Tsunami Warning Center in 1949.
The highest tsunami wave ever recorded occurred as a result of a catastrophic landslide into a bay. On July 9, 1958, an 8.3 magnitude earthquake caused 30 million cubic meters of rock to fall into the inlet of Lituya Bay in southeastern Alaska. Focused by the narrow width of the inlet, the wave rose to over 524m (1,720ft), stripping all trees below that height from the borders of the bay. The wave killed only three people on an island in the bay and two men in a fishing boat.
A similar event occurred in 1980. Mount St. Helens, an active volcano in Washington State, erupted explosively on May 18. The top 400m of the mountain disappeared and was replaced by a mile-wide horseshoe-shaped crater. The eruption also triggered a massive pyroclastic avalanche, which fell into Spirit Lake, below the mountain. The resulting wave was 260m high, as judged from the high-water mark on the opposite hillside.
Most tsunamis are caused by submarine earthquakes, and to this date, no one is able to predict with any certainty when an earthquake and its associated tsunami will occur. However, an earthquake can be detected almost immediately by sensitive seismographs because seismic waves travel around the world at about 14,000km/h. In contrast, a tsunami travels at less than 1,000km/h. Therefore, there can be a delay of several hours between the earthquake and the arrival of a tsunami at some distance. This delay could allow a population under threat to be warned in time to escape to high ground. But such a potential saving of lives requires constant monitoring with a widespread system of sensors.
After the 1946 Aleutian Islands tsunami destroyed the waterfront of Hilo and killed 165 people, the U.S. government organized the Pacific Tsunami Warning System. It consisted of the existing seismograph at the Honolulu Geo-magnetic Observatory and a new network of sirens that was spread throughout the Hawaiian Islands and coupled by telephone to the observatory.
Then in 1960, following the huge Chilean earthquake and its disastrous tsunami, 26 nations around the Pacific Ocean agreed to join in establishing the Tsunami Warning System. This was accomplished in 1965 under the auspices of UNESCO. Two centers were established to collect and analyze earthquake signals, determine whether a tsunami is impending, estimate the path of the tsunami, and if necessary, issue appropriate warnings. The U.S. National Oceanic and Atmospheric Agency (NOAA) operates the two regional centers. One is at Ewa Beach, Hawaii; the other is at Palmer, Alaska. The characteristics of an earthquake are recorded by seismographs at the two centers, by the U.S. Geological Survey’s National Earthquake Information Center, and by international stations.
The other leg of the Tsunami Warning System is a fleet of instrumented buoys that signal the passage of a tsunami. Beginning in 2000, NOAA distributed seven buoys around the perimeter of the Pacific Ocean. These buoys were the first in the Deep-Ocean Assessment and Reporting of Tsunamis (DART) system. With additions by the DART partners, the Pacific system has expanded to more than 50 buoys, with a few sprinkled in the Atlantic and Indian Oceans. Ideally, one would want more buoys out in the center of the Pacific to be able to monitor a tsunami as it sweeps across the ocean.
The idea behind the buoys is that the average water pressure near the bottom of the ocean is hardly affected by the rapidly fluctuating, wind-driven waves at the surface. The bottom pressure merely reflects the height of the water column. But any long, slow change in the average pressure, beyond a set threshold, can indicate the passage of a tsunami.
Each DART II buoy (the improved version of DART I) is anchored to the ocean bottom and is equipped with a bidirectional communication system. A pressure gauge and a battery-powered electronic package lie on the bottom near the buoy. Water pressure is recorded continuously and averaged in 15-minute blocks by the electronic unit. The pressure gauge is extraordinarily sensitive; it can resolve a 1-mm change in the 4-km height of the water column. The data averages are transmitted to the buoy at the surface by means of an acoustic link and then sent on to one of NOAA’s Geostationary Operational Environmental satellites and to ground stations. From there a warning can be distributed to various national centers. For example, during the February 27, 2010, tsunami at Chile, the average water column rose by 20cm over a period of 20 minutes as the crest arrived at a buoy and then decreased as the trough passed by. A warning was issued shortly afterwards.
The seismic and water pressure observations are used to sort through a computerized generation and propagation database and to select the most likely scenarios. This step requires only a few minutes to complete. Then these deep-water estimates are used as initial conditions to compute the propagation of the tsunami and its height when it arrives at selected coastal areas. This step can be completed in about 10 minutes. At especially vulnerable sites the shape of the bottom and slope of the shelf are taken into account. In recent years a statistical or probability technique has been used to estimate the range of hazards that a chosen coast can experience. Warnings are sent out by the member state and local governments, who spread the word by radio and television. NOAA’s Weather Radio System, which is directly available to the public, is also used to warn the public.
The system performed very well for the United States during the March 2011 tsunami in Japan. The wave reached Kona, Hawaii, after 7 hours with a height of about a meter. It caused significant flooding but no casualties. It touched the yacht harbor at Santa Cruz, California, in 14 hours, causing minor damage to boats and the dock.
Prediction techniques are constantly improving, thanks to Vasily Titov and his team at NOAA’s Center for Tsunami Research in Seattle, Washington. Not only can the height and time of arrival be forecast, but also the impact on a coastal zone. Titov was especially proud of the performance of his computer model during the April 11, 2012, tsunami off the western coast of Sumatra. This wave was generated by a magnitude 8.3 earthquake 437km southwest of Banda Aceh, a town made famous in the great tsunami of 2004. Although the earthquake caused widespread panic in Banda Aceh, the model accurately predicted that a tsunami was not a serious threat to the town. In contrast, the model severely underestimated the tsunami’s height at Hanimadhoo in the Maldive Islands. Tidal gauges there showed that the arrival time was accurately forecast but that the model failed to predict the 40-cm height of the wave. Fortunately, such a weak wave caused relatively little damage.
The terrible loss of life following the 2004 Indonesian earthquake and tsunami sparked an effort to set up a tsunami warning system for the Indian Ocean similar to that for the Pacific Ocean. UNESCO organized the system, which began operation in June 2006. It consists of 26 seismic centers, linked by satellite communications. The system performed adequately just one month later, on July 17, 2006, when a 7.7 magnitude quake and a 3-m tsunami occurred off the southwest coast of Java. The Pacific Tsunami Warning Center issued a tsunami warning about 12 minutes after the earthquake, but it was not always relayed in time to all populations in harm’s way, since there was concern about “not alarming the people unnecessarily.” Consequently, about 700 people died in Java because of the 2-m waves that swept inland at least 200m. No system can protect people who live close to an epicenter or who are not warned in a timely manner.
As we have seen, the existing tsunami detection system depends on a fleet of DART buoys along the earthquake-prone coasts of the Pacific. There is now a proposal for a new detection technique that might be able to identify tsunamis on the high seas. It would involve the Global Positioning System (GPS) and receivers on commercial ships. Several scientists have suggested the idea independently. Here is the story of how one of them, John Foster of the University of Hawaii, got the idea.
In 2010 Foster was a member of the crew of the oceanographic research ship Kilo Moana. On February 27, while the ship was in transit from Hawaii to Guam, a magnitude 8.8 earthquake occurred at Maule, Chile. Foster and his colleagues were able to analyze the continuous GPS recordings of the height of their ship above mean sea level. That allowed them to detect the Chilean tsunami, although the wave was only 9cm high. Foster was astounded. Nobody had expected to be able to detect a tsunami at sea because its height is usually so small. But the GPS location of the ship in three dimensions was sufficiently precise, with a resolution of just a few centimeters, to allow detection. The long wavelength of a tsunami (tens or hundreds of kilometers) is the key to distinguishing it from wind-driven surface waves.
Foster visualizes a network of shipborne GPS recorders linked by satellites that could detect tsunamis anywhere over the Pacific. As of March 2012 Foster and his colleagues were planning to test the feasibility of the scheme by linking two ships. Their plan was to stream GPS data continuously to determine the height of the sea in real time with sufficient accuracy to detect a tsunami. We shall have to see whether they are successful.
Tsunamis continue to inspire awe, fear, and morbid fascination in the general public. The opportunity of watching one in action on television or streaming video is irresistible. But for the people vulnerable to them, better warning systems—and the education to heed the warnings—are crucial to minimizing the devastation caused by these nightmarish water monsters.
